Among various kinds of sensors heretofore known for detecting temperature, a type of temperature sensor that uses a thin film thermistor element as a temperature-sensing element is disclosed, for example, in Japanese Patent Reissued Publication, No. H08-835932, and the like. FIG. 19 is a schematic cross sectional view depicting a structure of a temperature sensor of such kind. A temperature-sensing element 51 comprises a thin film of sintered ceramic formed on a substrate. Connecting parts for electric resistance of the temperature-sensing element 51 are formed of platinum film, and connected to lead wires 65 and 66 made of platinum. The lead wires 65 and 66 are housed within a metal housing 63 of a cylindrical shape, while being isolated with each other by electrical insulating glasses 61 and 62. The cylindrical metal housing 63 is fixed to a flange 64. The temperature sensor detects temperature electrically as a resistance of the temperature-sensing element 51.
In the case of adapting a sensor of this kind for an exhaust gas purification system of automobile, as has been studied for the purpose of environmental protection, the sensor can constitute a characteristically suitable temperature sensor, including reliability. However, it makes an extremely expensive temperature sensor in respect of cost effectiveness, since it uses platinum wires for the lead wires 65 and 66. Because the platinum wires actually used for one temperature sensor has weighed approximately 0.54 gr, the temperature sensor has cost approximately 800 yen for the platinum alone based on the platinum price (approx. 1,500 yen/gr) as of July, 1997. The prior art techniques thus have had a problem of making temperature sensors very expensive.